In magnetic disc drives, a voice coil motor is used to position the transducer heads over a desired radial position on a stack of magnetic discs that store information. When the disc drive is energized and the discs are spinning, the voice coil motor positions the heads over data stored on the spinning discs. The spinning generates an air bearing separating the heads from the spinning discs. When the disc drive is de-energized and the discs stop spinning, there is no air bearing and the heads contact the smooth stationary discs. If the sticking friction (xe2x80x9cstictionxe2x80x9d) between the heads and the discs is too great, the spindle motor may be incapable of rotating the discs when the disc drive is restarted. A common method of avoiding this problem is to move the heads with the voice coil motor to a xe2x80x9cparkxe2x80x9d portion of the discs when the disc drive is de-energized. The park portion of the disk is textured so that it will not stick to the heads, and no data is stored on the part portion. Various kinds of latches are used to latch the voice coil motor in this park position when the disc drive is de-energized. A magnetic latch shown in U.S. Pat. No. 5,734,527 to Reinhard, for example, comprises a latch with magnetically soft stainless steel balls that interact with the fringe fields in an air gap of the voice coil motor and provides one magnetic ball for latching, and second magnetic ball for unlatching.
As voice coil motors are designed smaller with narrower air gaps, the diameter of a detent ball that will fit in the gap is correspondingly reduced. When the air gap and the ball diameter are reduced by about 25%, the volume or mass of magnetic material in the ball goes down by about 58%. The detent torque, which is related to the mass of magnetic material in the ball, drops off with the smaller ball. The detent torque becomes lower and is active over a narrower range with a smaller detent ball. The magnitude and shape of the torque over a rotational range becomes inadequate to hold the voice coil in the latched position under conditions of vibration. A magnetic latch detent and latching method are needed that will fit in narrower air gaps and that can be shaped over the rotational range to provide adequate detent torque.
A first latch for a voice coil motor in a disc drive is disclosed. The latch includes first and second magnetic detents that are active in corresponding first and second rotational segments, offset from one another, in a rotational latching range. The selected offset shapes the detent torque over the latching range. The latch has a hub coupled to the magnetic detents and includes a latch surface that latches the voice coil motor. The detent torque is shaped by the offset to provide torque over a selected rotational range. The detent torque is increased by the use of multiple magnetic detents. Smaller detents can be used that fit in a narrower air gap and collectively provide adequate detent torque.
A magnetic detenting arrangement is also disclosed. A hub couples to the detenting arrangement and has a latch surface. The latch surface has a rotational latching range in which the latch surface latches the voice coil motor. The detenting arrangement includes a first magnetic detent that is active when the latch surface is in a first rotational segment in the latching range. The detenting arrangement also includes a second magnetic detent that is active when the latch surface is in a second rotational segment of the latching range. The second rotational segment is offset relative to the first rotational segment. The offset shapes the detent torque over the latching range.
A method of latching a voice coil motor is also disclosed. A latch having a hub and a latch surface is rotationally mounted. The voice coil motor is latched to the latching surface in a rotational latching range of the hub. The hub is magnetically detented relative to voice coil motor magnets with a first magnetic detent that is active when the latch surface is in a first rotational segment in the latching range. The hub is magnetically detented with a second magnetic detent that is active when the latch surface is in a second rotational segment in the latching range that is at a rotational offset from the first rotational segment. The offset shapes the detent torque over the latching range.